Patient interfaces, such as masks for covering the mouth and/or nose, are used for delivering gas to a patient. Such gases, like air, cleaned air, oxygen, or any modification of the latter, are submitted to the patient via the patient interface in a pressurized or unpressurized way.
For several chronic disorders and diseases, a long-term attachment of such a patient interface to a patient is necessary or at least advisable.
One non-limiting example for such a disease is obstructive sleep apnea or obstructive sleep apnea syndrome (OSA). OSA is usually caused by an obstruction of the upper airway. It is characterized by repetitive pauses in breathing during sleep and is usually associated with a reduction in blood oxygen saturation. These pauses in breathing, called apneas, typically last 20 to 40 seconds. The obstruction of the upper airway is usually caused by a reduced muscle tonus of the body that occurs during sleep. The human airway is composed of walls of soft tissue which can collapse and thereby obstruct breathing during sleep. Tongue tissue moves towards the back of the throat during sleep and thereby blocks the air passages. OSA is therefore commonly accompanied with snoring.
Different invasive and non-invasive treatments for OSA are known. One of the most powerful non-invasive treatments is the usage of Continuous Positive Airway Pressure (CPAP) or Bi-Positive Airway Pressure (BiPAP) in which a patient interface is connected to a pressure generator via a patient circuit including one or more tubes, wherein the pressure generator blows pressurized gas into the patient interface and into the patient's airway in order to keep it open. Positive air pressure is thus provided to a patient by means of the patient interface that is worn by the patient typically during sleep.
Examples for such patient interfaces are:                nasal masks, which fit over the nose and deliver gas through the nasal passages,        oral masks, which fit over the mouth and deliver gas through the mouth,        full-face masks, which fit over both the nose and the mouth and deliver gas to both, and        nasal pillows, which are regarded as patient interfaces as well within the scope of the present invention and which consist of small nasal inserts that deliver gas directly to the nasal passages.        
The patient interface is usually positioned and donned to the patient's head using some kind of headgear. Wearing a patient interface can be uncomfortable, since for providing an airtight seal between the patient interface and the patient's face, the patient interface has to be worn with a sufficient level of pressure on the face. The patient interface also has to be correctly fitted to the patient's face, since a poor fit of the patient interface will lead to a loss of pressure which can negatively affect the therapeutic treatment.
Whilst the patient interface may be correctly fitted prior to sleep, during sleep the user may move into a position where mechanical pressure is exerted onto the patient interface, in particular to the front, protruding section of the patient interface. During sleep the position of the patient interface may, for example, be disturbed by a pillow or other external obstacles. A user turns and the pillow or other external obstacle moves the patient interface, such that leakages may occur at the interface between the patient interface and the patient's face which results in a pressure loss via the ill-fitting sections. However, the provision of pressure in the patient's airway is one of the main purposes of such a patient interface. Hence, such a pressure loss will reduce the effectiveness of the therapeutic treatment. Another cause for a leakage may be the relaxation of the facial muscles as soon as the patient falls asleep. As a consequence, the stiffness of the facial surface changes and induces leakage of the patient interface.
A further problem that may arise when the position of the patient interface is shifted from its original, correctly fitted position is the possible formation of red marks in the patient's face.
Several attempts to solve the above-mentioned problems are known. US 2013/0118500 A1 discloses a method and system for managing a set of active headgear straps for a Positive Airway Pressure (PAP) mask including detecting a gas pressure value within the mask by means of a pressure sensor, analyzing the gas pressure value occurring in the mask with a processor for identifying an adjustment of the set of active headgear straps, and performing the identified adjustment with the set of active headgear straps. In other words, the headgear straps are adjusted in dependence to the gas pressure that occurs within the mask.
WO 2013/183018 A1 discloses a patient interface with a cushion element and an electro-active polymer material that allows to readjust the position of the cushion element upon activation of the electro-active polymer material. The electro-active polymer material is either controlled by means of a temperature sensor that measures the temperature at the interface between the patient interface and the patient's face or it is controlled by means of one or more pressure sensors which measure the pressure at different positions of the interface between the patient interface and the patient's face. The electro-active polymer material is activated if the temperature or the pressure increases above or falls below a certain threshold.
US 2004/0163648 A1 dicloses a gas mask for use with associated monitoring and controlling apparatus. Different types of sensors on or in the mask and straps or caps are therein used for monitoring of patients with sleep disorders, breathing disorders or for anesthesia. These sensors include oximetery sensors, patient position sensors, eye movement sensors, leak detection sensors, EEG, EMG, EOG, ECG, PTT, microphones, pulse, blood pressure, oxygen saturation, temperature, movement sensors, position sensors, light sensors, leak detection sensors and gas delivery sensors.
With regard to the above-mentioned problems there is, however, still room for improvement.